1. Field of the Invention
This invention relates to a resin-coated paper support material for photographic coatings.
2. Brief Description of the Background of the Invention Including Prior Art
As a result of the high demands of modern developing techniques and processes, resin-coated photographic paper supports have been developed which are highly resistant to penetration by developing and fixing fluids and which must possess certain other mechanical properties such as a certain stiffness, internal bond strength, breaking strength, etc.
The use of paper, coated on both sides with water-resistant resin, as a support material for photographic coatings, is not new. Such a material normally consists of a paper core to which a polyolefin coating has been affixed on both surfaces. The front side coating, onto which a light-sensitive emulsion coating is to be affixed at a later stage, usually contains a white pigment at least, in order to increase the definition of the photographic image. Other additives, such as dispersing agents, antioxidation agents, and antistatic agents, as well as color pigments, may be found.
Although this effectively protects both surfaces of a base paper against the penetration of a developing solution, it leaves the edges of such exposed. As a result, air and light, discoloring the developer, penetrates these edges and the paper's value as a photographic base material suffers.
In order to minimize this edge penetration of the photographic base paper, it is "hard-sized". Apart from the internal water-repellent sizing of the paper core, further tests have been carried out to size the surfaces of the base paper by coating them with differing high-molecular substances, among which are to be found gelatine, oxidized starches, and other starch derivatives, carboxy-methyl cellulose, modified polyvinyl alcohols, and other binding agents. This surface sizing of the base paper is done with aqueous coating solutions which, as in the case of the above-mentioned binding agents, can also contain further additives, such as optical brightness, pigments, defoaming agents, cross-linking additives, etc. The solutions are applied to the surface of the paper either by a sizing press or by other spreading processes such as blade-coating method, rod-coating method, or roll-coating method.
Starch has been applied for a long time to improve, above all, the surface characteristics and as additional protection of the surfaces of the beater-sized base paper against outside influences. The coating usually contains modified, degraded starches, such as cationic, anionic, or oxidized starches.
The application of starches in surface treatment (German Patent No. DE 25 15 823), while ensuring good adhesion between the base paper and the polyolefin coating, leaves something to be desired regarding edge penetration. Moreover, it is unsatisfactory regarding other physical values, especially internal bond strength.
The German Patent No. DE-OS 32 41 599 suggests sizing the paper core by applying a coat of dicarboxylic acid modified polyvinyl alcohol. This method, however, does not ensure good uniform adhesion between the polyolefin coating and the paper core, and the so-called edge penetration is equally unsatisfactory. This method, moreover, is not without its problems because of the low electrolytic tolerance levels of the polyvinyl alcohol.
The attempt to use a monocarboxylic acid modified polyvinyl alcohol, as taught in German Patent No. DE-OS 3,543,597, while offering good results as far as edge penetration, inner rigidity, and polyolefin adhesion are concerned, presents problems during the manufacture of the solution. In the presence of salts, the polyvinyl alcohol, as a result of its low electrolyte compatibility tolerance, tends to flocculate.
The precipitate, formed by the application of salts to improve conductivity, is extremely difficult to redissolve during the mixing process of the solution. The result is a reduction of the surface quality of the photographic base paper when such a coating is applied, caused by the above-mentioned precipitate creating unevenesses on the surfaces. On the other hand, the precipitate removal by filtering causes a reduction of the amount of binding agent and, therefore, reduces the values of the internal bond strength of the base paper.